Dielectric resonator based passive wireless sensors

Sensing in harsh environments is a problem of great interest to the aerospace, automotive and the process control industry. However, for the dream of ubiquitous sensing to be fully realized, it is critical that the financial and environmental cost per sensor be minimized. Passive wireless sensing offers a means to measure quantities of interest without having the designers and users meticulously plan wiring installations and battery replacements for an application, resulting in an increase in functionality and lifespan of the system under test. Contemporary passive wireless sensors, based heavily around surface acoustic wave technology have also been shown to perform at high temperatures. In this work we attempt to meet a similar goal by using a much simpler device, the dielectric resonator, as a sensor. This thesis provides a detailed theoretical study of the use of isolated dielectric resonators for wireless passive sensing applications. The resonator itself works as both a sensor and as an antenna. The scope of dielectric resonator in sensing applications was already developed but limited knowledge was available on its performance as a radiating antenna. The information on the radiated fields, resonant frequency and radiation Q-factor of various modes was not reported in literature in a formal manner. To achieve this target, a novel and innovative concept for wireless passive sensing is presented, with focus on an industrial application of dynamic torque, force and high temperature. The measurement concept is based on the harmonic perturbation of a dielectric resonator caused by two physical phenomenon, an applied force that results in a strain changing the boundary conditions and an applied temperature that results in a change of dielectric properties of the resonator that results in a frequency perturbation. Numerical modelling of the Zirconium tin titanate (ZST) based isolated dielectric resonator (ϵr = 37) in 1, 2 and 3- dimension is performed for this purpose using two very powerful techniques of computational electromagnetics i.e. Boundary element method and Finite element method. Based on these solving techniques, a general purpose numerical solver based on Maxwell and Helmholtz’s wave equations for electromagnetic field analysis of a dielectric resonator is developed in MATLAB and results are compared with FEM simulations from COMSOL and Ansys HFSS. Frequency, quality factors and the radiation patterns are calculated and simulated. Circular symmetry is studied for torque and temperature sensors corresponding to a 2.5 D model in COMSOL. For the force sensor, a full 3D model was developed and optimized for peak radiation performance. The fundamental transverse electric TE01δ mode in a metallic cavity is either coupled with a magnetic loop coupler connected to an external antenna or used directly with the radiation coupling from the edge of the parallel plate. Some higher modes, like the HEM31δ mode exhibit a good balance between the radiation coupling and transient energy storage of the incoming electromagnetic wave. Due to the six-fold symmetry of the HEM31δ mode, a sectioned cylindrical transponder is also studied. Several experiments are set up to characterize the three types of sensors with resonators in two configurations, namely a parallel plate dielectric resonator and an isolated dielectric resonator on a dielectric support. For the torque sensor, a microstrip coupler with compensation capacitors allows for a rotational invariant near field coupling between the stator and the torque sensor. The high loaded Q of the resonant sensor, results in a precision of ±0.5Nm over a range of 3500Nm. The frequency sensitivity was shown to be 3.9 ppm/μϵ. The extraordinary performance of the sensor is demonstrated in a custom-built four-square test rig under dynamic torque and shear force loading. To characterize the high temperature sensor, high radiation-Q modes are experimentally verified for far field coupling. Radiation patterns studied in a laboratory and an outdoor environment, are in good agreement with the simulations with loaded quality factors of up to 3000. The radiation plots obtained show a directivity of about 7.5dBi at 2.5 GHz. The sensor is placed in a ceramic oven, with the readout antenna outside at room temperature. Wireless temperature measurements of up to 700 ◦C with a resolution of 0.5 ◦C are experimentally demonstrated. By using different high Q EM modes in a parallel plate configuration, which show an linearly independent temperature characteristic, it can be shown that a mean square error of under 4 ◦C is possible even without any sensor calibration. A force sensor is realized in an open ended waveguide configuration with sensitivity of 20 ppm/N and a wireless readout distance of some meters. Cross sensitivity of the resonant frequencies of the modes w.r.t humidity, temperature and strain are also characterized. Industrial application of this new class of wireless passive sensors with integrated antennas is imminent. The research is chiefly driven by the desire to simplify the device and reduce costs. It is exceedingly clear that the DR is the simplest of them all.